Beogram 4002: Replacing the Detector Arm Light Bulb with a LED

The Beogram 4002 that I am currently rebuilding also needed a new light bulb in the detector arm. The previous owner had done a poorly executed 'repair' attempt, and ripped out two wires from the detector arm lamp assembly, while also bending the photo cell backwards - unbelievable what some 'experts' are coming up with. Here is a picture of the assembly as I saw it the first time:

For comparison I pulled the compartment out of the detector arm of my Beogram 4004, which is in a pretty good condition:

These light bulbs are hard to come by and I also believe in LEDs as a way to get these units stable for another 30 years. So I decided to replace the light bulb with a small 3 mm LED.

This effort turned into an interesting experience, since on first try, I was not able to get the record detection mechanism working after I put in the LED with a 220 Ohm resistor. The turn table simply ignored any record that I would put on its platter.

Study of the circuit revealed that the original designers of this gorgeous piece of analog control electronics proofed the circuit against failing light bulbs (a failed light bulb would result in always detecting a record whether there is one or not, with potentially fatal consequences for the pickup). The much lower current in the LED resulted in a partial activation of this fail safe mechanism, which prevented the Beogram from auto-lowering its arm. This was fixed by replacing R36 with a 560 Ohm resistor, which resorted the original balance in the circuit.

I made a video about the entire process. The video discusses the detector arm circuit and explains how the LED exchange was performed:

Here is a photo of the final result:

Beogram 4002: Conversion of the Tracking Sensor Light Bulb to an LED and Adjustment of the Sensor

I started working on a Beogram 4002 that I obtained locally from Craigslist about a year ago. I thought it would be a nice practice run before I give the cherished Beogram 6000 4-Channel a go that I recently found.

I plugged it in after opening it up in the hope to see the location of the smoke should any develop...;-). No smoke, but no tracking either. Pretty much nothing worked, no tone arm lowering etc...also some traces of amateur repairs.

I determined that the light bulb in the tracking sensor was out, and I decided to replace it with an LED since the bulb cannot easily be replaced since it is glued into the sensor housing, i.e. cannot be separated. I also am a fan of LEDs since they have a much better long-term stability, especially should the unit be shipped at some point.

I ended up using a 3D printed part to hold the LED and the current limiting resistor. The part essentially replaced the light bulb fixture on the sensor housing. Here is a picture of the end result:

I made a video that explains the whole procedure including the adjustment of the tracking mechanism for the right tone arm angle relative to the record grooves. The video also discusses how the tracking mechanism works. Enjoy!:

On to the dead sensor arm!

Beocord 5000 (4715/4716): Fader Fixed and Putting it Back Together

I looked into the Fader of the Beocord 5000 (4715) that is (was, hopefully) on my bench. It turned out that the contacts of the Fade In and Fade Out control buttons were strongly corroded. They simply did not make contact. I pulled a strip of paper coated with DeoxIT D100 through the contact pads, while pressing the buttons, and this cleared things up. The paper had nice black streaks of them, indicating a fair amount of oxide removal. I did the same with the other buttons, giving this deck an as-new user feel. Swift response whenever a button is lightly pressed...very nice!

This encouraged me to put the deck back together. But first I needed to re-glue the veneer on one of the corners of the wood panels. I used some wood glue that I carefully inserted in between the veneer and the particle board using a thin needle. This allowed me to avoid bending the veneer to much. Here are a couple of pics of the procedure. I used snippets of transparency foils to prevent accidentally glueing on the cardboard pads that I used to protect the wood from damage by the carpenter clamps.

Once the corner was stabilized I put the wood panels back on the BC5000, and connected it to the Beomaster 6000 for a first tryout with the Beogram 4004. I taped Water Babies by Miles Davis, one of my all-time favorites of my vinyl collection. This went very well. There is almost no difference between the recording and the original. Also, the 'infamous' double capstan mechanism does not seem to have a problem...no eaten tapes so far, and the recordings seem to be very consistent. Here are a couple impressions of the kit in action:

And a somewhat shaky GIF of the entire set-up while taping (it is pretty long and a linear sweep is pretty difficult with a cell phone...;-):

Beocord 5000 (4715/4716): First Recordings!

Today I finally hooked up the still open Beocord 5000 to my Beomaster 6000 4-Channel for some basic testing. I made a recording on a new TDK SA 90 via my Beogram 4004 (Miles Davis: Porgy and Bess, a nice pressing from the 70's; CBS 32188). It worked beautifully! The Beocord worked and responded like new (well, almost...see below). I pressed the record button, and adjusted the level sliders to yield a signal just below the red indicators, then re-set the tone arm to the beginning of the track ("The Buzzard Song"), and pressed Start. The 5000 started recording. After the song was over, I stopped and rewound, and played back. It was perfect. I am quite amazed of the quality of this 1977 deck's reproduction. Almost no difference, just a bit more hiss (I did use Dolby NR, though). Here is a first impression of the deck in action with and without the plexiglass cover:

What a beautiful machine! I just love the internal layout...wonderful analog pre-microcontroller design work. A slight downer of this blissful moment: Unfortunately, further playing around revealed that the Fade function is not (yet) working...well, tomorrow is another day! I am sure I will get this figured out, too!

Beocord 5000 (4715/4716): Slow Rewinding, Inconsistent Pause Function and Auto Stop

After putting the drive unit back into the enclosure of the Beocord 5000 (4715) I started testing the basic functions. On first glance things were not too bad. It fast forwarded, rewound, played and stopped. However, after playing with it for some time I realized that the Auto Stop function would only work for a few minutes and then quit. The tape would reach the end and the drive mechanism would not quit...probably not good for the motor! Only turning the deck off for an hour or so would bring it back.

I also realized that the Pause function often would not work, i.e. behave inconsistently. Further experimenting showed that rewinding was considerably slower than fast forwarding, and that at the end of the rewind the speed would drop to levels that made me concerned it would just stop. Not what I expected from such nice two-motor drive mechanism. Also, I could not understand why the speeds should be different...practically the same friction should occur (except that the pulleys in the free wheeling mechanism that reverses the winding direction change - but I had lubricated them with a small drop of Ballistol when I took the drive apart, so they are pretty low-friction). Also, manually cranking the motor suggested that there was not much of a difference between directions in terms of torque, i.e. friction could not explain the difference.

So I started investigating. After much measuring this and that and comparing with the voltages stated in the circuit diagram I finally realized that one of the transistors in the Pause flip-flop (3TR27) seemed to have a problem. Its base measured -3.8V when paused and -10.8 when running. It should be -10.2V vs. -10.8V considering the Si turn on voltage of this transistor. And indeed, its sister transistor 3TR26 showed these values. I replaced TR27 with a 2N2222 NPN type and tried again. Low and behold, this fixed all issues. Out of a sudden Pause and Auto Stop were consistent and the reversing speed was normal. After examining the BC548B that I extracted, I had to come to the conclusion that most likely a bad solder joint may have been the problem - the transistor tested o.k...but of course it is difficult to tell...it also may have a heat related issue which only shows up after some time...Anyway, now it works! Analog networks! Definitely interesting to work on them.

For the record: After the repair, the measured winding times for a brand new TDK SA90 tape were about 85 sec in both directions. The auto stop time from the moment the tape would stop was ~7 sec.

Before exchanging TR26, rewinding took about 120 sec, while fast forwarding took about 85 sec. Auto Stop, when it worked was also ~7 sec.

Along the way I did some measurements on the Auto Stop function part of the circuit since initially I suspected the problem there. Initially, I did not understand the terminology in they 1977 circuit diagram.

The 0 to -10.9V step at the collector of 3TR12 is actually meant to be a pulse. The >> << is apparently the indicator for this (Initially I thought it was some 'secret' reference to Fast Forward and Rewind..;-). So in the beginning I connected my multimeter, and did not see the expected voltage jump. Finally, hooking up my oscilloscope yielded the pulse:

It is only about 10-20 us long while charging 3C5 (100nF) via 3R15 (120R). This makes sense since the time constant of this combination is just about 10us. But this is enough to briefly tug the base of 3TR4 and initiate the Stop function via 3TR5.

I also measured the pulse at the base of 3TR12. It looks like this (scale is from -11 (base line) to -10.4V (top of the peak)

It is interesting to note that connecting the oscilloscope or a multimeter to either side of 3R41 (1M) resulted in a deactivation of the Auto Stop function due to the impact of the internal impedance of the measurement instruments. This network is pretty high impedance, i.e. one needs to be alert with regard to impedance impact on measurements and performance...

Time to try making a recording. Haven't done this in a few years, maybe I'll even make a mix tape...;-).

Beocord 5000 (4715/4716): Replacing the Belts

I received the Beocord 5000 belts from 'Dillen' (Beoworld.org). And so I was finally able to continue working on the unit. The procedure is fairly straight forward, but needs some patience, since the drive mechanism needs to be dismantled considerably to get at the belts. Furthermore, there is only a discussion how to get to the belts on the top side of the drive in the service manual, but not how to get to the ones on the bottom. However, there is an interesting thread on Beoworld.org where parts of this process are discussed in some depth. This discussion also contains a bit of a horror story about breaking PCB #3 ('Electronic Switch') in half when trying to get the belt in that drives the counter seated in that board. Anyway, after studying this thread and reading the Service Manual, I carefully set out to do it.

I made a video about the process, which shows most of the steps in some detail (****update: I further streamlined the procedure for extracting the tape mechanism. See here.*****):

After putting the drive back in, it seemed that most of the Beocord 5000 was working. But further testing is in progress.

I thought it was interesting to log the dimensions of the old belts and the new ones in comparison (the numbers refer to the numbers given in the Service Manual, Sec. 4).

Belt	Old ID	New ID	Old Crossection	New Crossection
Belt 119 (flywheels/spools)	3.75”/95 mm	3.3”/84.3mm	0.075”/1.9 mm, round cs	0.08”/2 mm, round cs
Belt 115 (DC motor for flywheels to pulley)	1.86”/47 mm	1.75”/44 mm	0.06”/1.5 mm, triangular, height=base	0.05/1.3 mm, square cs
Belt 143 (Capstans) (flat belt, thickness ~0.5 mm)	7.6”/194 mm	7.05”/ 183 mm flattened length	0.185/4.7 mm width	0.2” or 5 mm width
Belts 132/135 (Counter)	2.2”/56 mm	2.05”/ 52mm	0.035”/0.9 mm round cs	.04”/1 mm, square cs

The comparison shows that the old belts were quite a bit elongated. Notable are the differences in crossections. The old #115 belt had a triangular crossection, but the square replacement seems to work fine. Same with the counter belts, which were round in the original configuration.

An Early Christmas! Beosystem 6000 is in the House!

I went for a little drive today (about 340 miles...;-) and picked up a pristine, almost new looking complete Beosystem 6000 from another vintage audio enthusiast!!! What a lovely day! Beogram 6000 with MMC6000 in great cosmetic condition. All the keypads are practically new-looking, veneer is great...even came with replacement plexiglass parts and a full set of owner manuals, service manuals (even the 'product description' or 'Technische Produktbeschreibung' as it is in German for the Beomaster 6000!). Incredible! Of course everything needs rebuilding, but definitely this is an early Christmas present for the Beolover!!!...;-). Here are a couple pics: